Data transmission systems are known to include terminal devices (sometimes called readers or exciters) and portable data devices (sometimes called cards or smart cards). It is well understood that today's portable data devices include memory and processor devices that require power from the terminal device. Once such a portable data device (which may be contactless or contacted/contactless--sometimes referred to as combi-cards) enters into the excitation field of the terminal device, power and data can thereafter be transferred from the terminal device to the portable data device. Many factors have an effect on the apparent power seen by the portable data device. In particular, varying proximity to the terminal device and different applications/transactions have an impact on the power being seen and consumed by the data device. These varying power levels cause operational problems in the cards, as later described.
Depending on the card function being exercised at any particular time, the amount of DC current required by the card can vary. For example, if a simple state machine is all that is required for an access control, read-only mode of operation, the card might only draw on the order of 300 .mu.A at 3 V. If a more complex transaction is initiated, such as an electronic purse debit for a vending machine or a bus token, a microprocessor or microcontroller may need to be activated, and the current draw might go up to 1 mA or higher, depending on the complexity and clock speed of the processor. Such a transaction would also require, at some time, a memory write or erase, and this mode might draw an additional 500-800.mu.A of current. If a very secure mode is required, such as a high value transaction or a high security building or room access, an encryption or authentication algorithm is commonly employed. Such functions are computationally intensive, and completion in a timely fashion generally requires auxiliary processing power. These auxiliary computational modes could increase the current draw in the card by 5 mA or more, depending on the clock speed and complexity of the implementation.
For a given card--reader separation, the current flowing in the reader antenna has to be above a certain level to provide sufficient power to the card so that, after rectifying the coupled energy, the card's DC current draw requirements are maintained. A competitive advantage is enjoyed by cards that can function at greater distances from the reader. Reader manufacturers generally drive the antenna with as much current as is allowed by local radiated emissions regulations. Larger reader currents also mean larger unintentionally-radiated far-fields, which could interfere with other frequency bands in the vicinity. This is one of the reasons that the 13.56 MHz world-wide Industrial, Scientific and Medical (ISM) band is being considered by contactless card standards bodies, e.g., ISO-14443. This particular region of the spectrum allows large radiated emissions for high power, narrowband applications, as described with reference to FIG. 1.
FIG. 1 shows a spectral diagram 100 that includes a spectral mask 102 depicting the FCC regulatory emission limits for the ISM band. That is, the actual power-frequency response curve 104, which represents the power levels emitted from the terminal, may not exceed the limits shown in mask 102 at the frequencies shown. For example, under FCC part 15, a radiated E-field strength of 10,000 .mu.V/m measured at 30 m, is the maximum power level 110 allowed within .+-.7 kHz of 13.56 MHz. Similarly, outside of this narrow band, the radiated E-field must fall below the general limit 108 of 30 .mu.V/m as measured at 30 m.
To communicate with the card, the reader must impose a modulation on the antenna current. This modulation must be easily detectable at the card so that it can be demodulated with a low-complexity, low-cost, low-current drain card receiver. The value of the modulation index agreed upon in ISO-14443 is .+-.10% nominal about the mean carrier value for logical 1's or 0's. For random data, this level of modulation for reasonable data rates (105.9375 kbps in ISO-14443) will result in sidebands that are down about 25 dBc peak, 30 dBc average in the International Special Committee on Radio Interference (i.e., CISPR-16) 9 kHz measurement bandwidth. These spectral sidebands will fall outside of the .+-.7 kHz ISM window, so the largest they can be is 30 .mu.V/m at 30 m under FCC part 15. That means the largest the modulated carrier could be is 25 dB higher, or 533 .mu.V/m, even though up to 10,000 .mu.V/m is allowed for the unmodulated carrier.
For a state-of-the-art microprocessor card (1 mA at 3.3 Vdc), the amount of read range (i.e., maximum separation between terminal and portable data device) achievable with these levels of radiated emissions is only on the order of 10-12 cm. Any higher-current modes required by more sophisticated transactions would only reduce this read range.
Accordingly, there exists a need for a data transmission system that permits an activity-dependent power level to be present at the portable data device. Such a data transmission system that could dynamically provide such an increase in power without exceeding regulatory limits would be an improvement over the prior art.